implement connection-pool for sqlite

- just a casual implement for :memory:
- move some code-segment around(dal.__init__.py)
- unittest for sqlite passed.
fix an error

- we shouldn't assign to cls.__toresdo_db_conn_table__ directly. this
paramenter should only exist on class directly inherited from
AdapterBase. Once we assign it with None on some class not we expected,
that class would have its own __toresdo_db_conn_table__.
- don't init Model() directly (we don't want to create a 'Model' table.)